An optical disk unit is traditionally equipped with an optical pick-up device which is movable frontwards and backwards by means of a drive motor and a disk table which is rotatably driven by means of a spindle motor. An optical disk is placed on the disk table for rotation together therewith. As is generally known, the optical disk unit operates to read and reproduce a predetermined information signal recorded in a recording track through a record-tracking operation according to which the predetermined information signal is recorded by vertically emitting a laser beam generated from the optical pick-up device to the optical disk from its bottom, while transferring the optical pick-up device in a radial direction of the optical disk.
In such optical disk units, since the information signal recorded in the optical disk is read while transferring the optical pick-up device from the inner periphery of the optical disk to the outer periphery, there is a possibility that tracking error in the recording track results from error in transfer of the optical pick-up device and as a result the information signal is not read and thus a good reproduction cannot be made.
For the reasons indicated above, the driving-force transmission gear mechanism which is provided for transferring the optical disk unit, is conventionally equipped with a backlash eliminating mechanism, for eliminating backlash between a meshing pair of gears and for enhancing reliability in transfer of the optical pick-up device.
One such conventional optical disk unit will be hereinbelow explained in accordance with FIG. 39.
In this drawing, reference numeral 1 denotes a mechanical deck which is formed with a through window 1a being substantially rectangular from the plan view and opening in a vertical direction of the deck and a cut-out 1b being substantially semi-circular in the plan view and integrally formed with the through window 1a. The mechanical deck is disposed in a body (not shown) of an optical disk player.
Reference numeral 2 denotes a spindle motor which has an output shaft penetrating the cut-out 1b and is fixed on the reverse side (under surface) of the mechanical deck 1.
Reference numeral 3 denotes a disk table, on which the disk is placed, is fixed on the top of the output shaft 2a and has a driven connection with the spindle motor 2 for co-rotation therewith.
Reference numerals 4 and 5 denote a pair of right and left guide shafts respectively extending in the advancing and retreating directions of an optical head carriage as set out below. The guide shafts are arranged in parallel with each other in the body (not shown) of the player and fixed at the perimeter of the window 1a on the reverse side of the deck.
Reference numeral 6 denotes the optical head carriage which is provided for transferring an optical pick-up device 7 in the front and rear directions and slidably mounted on the guide shafts 4 and 5 through bushings 8.
Reference numeral 9 denotes a rack mechanism which is provided for transmitting a driving force produced by a motor. The rack mechanism consists of a first rack 10 extending frontwards and rearwards and fixed on one side of the optical head carriage 6, a second rack 12 extending frontwards and rearwards and slidably mounted on the upper surface of the first rack 10 through a spacer 11, and a compression coil spring 13 operably disposed between the first and second racks 10 and 12 for spring-loading them and serving as a backlash eliminating mechanism by producing the spring bias so that the two racks are spaced apart from each other.
Reference numeral 14 denotes a motor which is fixed on the upper surface of the mechanical deck 1 at one side of the latter for driving the head carriage. A motor gear 15 is fixedly attached to an output shaft 14a of the motor 14 in such a manner as to be placed at the reverse side of the mechanical deck 1.
Reference numeral 16 denotes a drive gear rotated by driving the motor 14. The drive gear is rotatably mounted on the reverse side of the mechanical deck 1 by way of a support shaft 17.
Reference numeral 18 denotes an intermediate driving-force transmission gear mechanism is provided for transmitting a driving force produced by the motor 14 to the optical head carriage 6. The intermediate transmission gear mechanism is mechanically linked to both the rack mechanism 9 and the drive gear 16 and consists of a driving-side transmission gear mechanism 19 and a driven-side transmission gear mechanism 20.
Of these transmission gear mechanisms 19 and 20 constructing the intermediate transmission gear mechanism 18, the driving-side transmission gear mechanism 19 consists of two gears 21 and 22 both being in meshed engagement with the drive gear 16 and being rotatably disposed, and a compression coil spring 23 serving as a backlash eliminating mechanism which acts to bias the respective gears 21 and 22 in their opposite peripheral directions.
Of these gears 21 and 22, the gear 21 is integrally formed with an upwardly extending small gear 24 at its central portion.
On the other hand, the driven-side transmission gear mechanism 20 of the intermediate transmission gear mechanism 18 consists of two gears 25 and 26 both being in meshed engagement with the small gear 24 and being rotatably disposed.
Additionally, the gears 25 and 26 are in meshed engagement with the first and second racks 10 and 12, respectively.
In the optical disk unit as set forth above, the driving force produced by the motor 14 is transmitted from the drive gear 16 through both the gears 21 and 22 of the driving-side transmission gear mechanism 19 and through both the gears 25 and 26 of the driven-side transmission gear mechanism 20 to the first and second racks 10 and 12 of the rack mechanism 9, with the result that the optical head carriage 6 can be moved along both the guide shafts 4 and 5 in the front and rear directions.
At this time, backlash between the drive gear 16 and the gear 21 and backlash between the drive gear and the gear 22 are both absorbed by means of the compression coil spring 23, while backlash between the gear 25 and the rack 10 and backlash between the gear 26 and the rack 12 are both absorbed by means of the compression spring 13. Additionally, the respective backlash between the small gear 24 and each of the gears 25 and 26 is absorbed by means of both the compression coil springs 13 and 23.
In the prior art optical disk units, since the two backlash eliminating mechanisms (the compression coil springs 13 and 23) are respectively provided in the rack mechanism 9 and the driving-side transmission gear mechanism 19, the prior art optical disk unit requires phase matching at respective toothed portions of the rack mechanism 9 and the driving-side transmission gear mechanism 19. This results in troublesome assembly task when totally assembling the respective mechanisms. Additionally, it takes a long time for assembly work.
Furthermore, since the compression coil -spring 13 is incorporated in the rack mechanism 9, the rack mechanism 9 is so designed as to have an undesirably larger dimension in its width direction. The above-noted larger dimension causes a long distance from the guide shaft 5 to the engaging point of the rack mechanism 9 and the driven-side transmission gear mechanism 20. As a result, when transferring the optical pick-up device 7, the moment of a force about the above-noted engaging point with regard to the optical head carriage 6 tends to increase, thereby increasing transfer loss in the head carriage.